1. Field of the Invention
The present invention relates to a conductive member used in an image forming apparatus such as a copying machine, laser beam printer and facsimile, a process cartridge having the conductive member, and an image forming apparatus having the process cartridge.
2. Related Art
There has been used a conductive member as a charging member, which performs a changing process to a photoconductor, photoconductive drum, or image carrier, or as a transferring member, which performs a transferring process to toner on a photoconductor, in an image forming apparatus of electrophotographic system such as a conventional electrophotographic copying machine, laser beam printer and facsimile.
FIG. 9 illustrates an image forming apparatus 120 of electrophotographic system having a conventional charging roller. The image forming apparatus 120 of electrophotographic system comprises a photoconductive drum 102 on which an electrostatic latent image is formed, a charging roller 102, which has contact with the photoconductive drum 101 to perform a charging process, an exposure device 103 such as leaser beam, a developing roller 104, which transfers toner to the electrostatic latent image of the photoconductor drum 101, a power pack 105, which applies DC voltage to the charging roller 102, a transfer roller 106, which transfers a toner image on the photoconductive drum 101 onto a recording paper 107, a cleaning device 108, which cleans the photoconductive drum 101 after the transfer process, and a surface potential meter 109, which measures the surface potential of the photoconductive drum 101.
This image forming apparatus 120 of electrophotographic system includes a process cartridge detachable system. More particularly, in the image forming apparatus 120 of electrophotographic system, the process cartridge 110 having the photoconductive drum 101, charging roller 102, developing roller 104 and cleaning device 108 can be detachably attached to the body of image forming apparatus. The process cartridge 110 needs to comprise at least the photoconductive drum 101 and charging roller 102. This process cartridge 110 is attached to a predetermined position of the body of image forming apparatus. Thereby, the process cartridge 110 is connected to a driving system and electric system disposed in the body of image forming apparatus. Moreover, a functional unit normally required for another electrophotographic process is omitted in FIG. 9 because it is not required in the present invention.
Next, the basic image forming operation of the conventional image forming apparatus 120 of electrophotographic process will be explained.
If DC voltage is applied to the charging roller 102, which has contact with the photoconductive drum 101, from the power pack 105, the surface of the photoconductive drum 101 is equally charged at high potential. After that, if the image light is irradiated to the surface of photoconductive drum 101 by the exposure device 103, the electric potential is decreased in the irradiated portion of the photoconductive drum 101. Such charging mechanism to the surface of photoconductive drum 101 by the charging roller 102 has been known as discharge according to Paschen rule in a micro space between the charging roller 102 and the photoconductive drum 101.
Since image light is distribution of light volume corresponding to white/black of an image, if the image light is irradiated, electric potential distribution, i.e., an electrostatic latent image corresponding to a recording image is formed on the surface of the photoconductive drum 101 by the irradiated image light. If the portion of the photoconductive drum 101 formed with such an electrostatic latent image passes through the developing roller 104, toner is transferred depending on the high-low potential, and a toner image that the electrostatic image is visualized is formed onto the photoconductive drum 101. A recording paper 107 is fed to the portion of the photoconductive drum 101 formed with the toner image by a resist-roller (not shown) at a predetermined timing, and overlaps the toner image. After this toner image is transferred to the recording paper by a transfer roller 106, the recording paper 107 is separated from the photoconductive drum 101. The separated recording paper 107 is fed via a feeding path, and is thermally fixed by a fixing unit (not shown). Thereafter, the recording paper 107 is discharged outside the body of image forming apparatus. If the transferring is completed as described above, the surface of photoconductive drum 101 is cleaned by the cleaning device 108, and also the residual charge on the surface is eliminated by a quenching lamp (not shown). Therefore, the image forming apparatus is ready for a next image forming process.
There has been known a contact charging method, which brings a charging roller into contact with a photoconductor drum, as a charging method using a conventional charging roller (reference to JP S63-149668A and JP H01-267667A). However, such a conventional contact charging method has following problems.
(1) A material comprising a charging roller exudes from the charging roller, and the material adheres to a surface of a body to be charged. Thereby, the charging roller mark remains on the surface of body to be charged.
(2) If direct voltage is applied to a charging roller, the charging roller, which has contact with a body to be charged, shakes, resulting in generation of charging sound.
(3) Since toner on a photoconductive drum is adhered to a charging roller (especially, toner is easily adhered by the above exuding), a charging performance of charging roller is reduced.
(4) A material comprising a charging roller is adhered to a photoconductor drum.
(5) When stopping a photoconductive drum for a long time, a charging roller is deformed permanently.
A charging device having a close charging method, which allows a charging roller to come close to a photoconductive drum, has been proposed as an art for solving the above problems (reference to JP H03-240076A and JP H04-358175A). In the charging device having this close charging method, the charging roller faces the photoconductor drum to be the closest distance (50-300 μm), and the photoconductive drum is charged by the voltage applied to the charging roller. In the charging device with the close charging method, since the roller does not have contact with the photoconductive drum, the material comprising the charging roller is not adhered to the photoconductive drum and the roller is not permanently deformed when the drum is stopped for a long time. Accordingly, the above problems of the charging device with the conventional contact charging method are solved. Moreover, in the charging device with the close charging method, the amount of toner to be adhered to the charging roller is reduced, so the toner on the photoconductive drum, etc., is unlikely adhered to the charging roller. Therefore, the charging device with the close charging method is a superior charging device.
In a charging device with a close charging method described in JP H03-240076A and JP H04-358175A, spacer ring layers are attached to both end portions of a charging roller in order to maintain a gap between the charging roller and a photoconductive drum. However, in the charging device with this close charging method, since an accurate gap is not considered, the gap between the charging roller and the photoconductive drum is fluctuated by variations in the dimensional accuracy of the charging roller and spacer rings. Thereby, the charging potential of photoconductive drum is fluctuated. Therefore, toner is adhered to a white background when forming an image; thus, an image error is generated.
In order to solve the above problem, there has been proposed a charging device including tape-based space holding members each having a predetermined thickness (reference to JP2002-139893A). However, if the charging device including the tape-based space holding members is used for a extended period, the tape-based space holding members are worn away, or toner enters between the charging roller and the tape-based space holding members and is fixed therebetween. Thereby, the gap is not maintained between the surface of photoconductive drum and the surface of charging roller. Moreover, in the charging device including this tape-based space holding members, a highly accurate gap is not formed because of variations in the thickness of tape-based space holding members.
Consequently, the present inventors have proposed a charging member 210. As shown in FIG. 10, the charging member 210 comprises a conductive supporting body 201, an electric resistance adjusting layer 202 formed on the conductive supporting body 201, and space members 203, 203 formed in both ends of the electric resistance adjusting layer 202. Each of the space members 203, 203 comprises thermoplastic resin, which satisfies durometer hardness: HDD30-HDD70 and abrasion mass of taber type abrasion tester: 10 mg/1000 cycle or less (reference to JP2004-354477A).
This charging member 210 comprises a structure that the space members (space holding members) 203 are pressed into both end portions of the electric resistance adjusting layer 202. In this charging member 210, the space members 203 are formed in the end portions of electric resistance adjusting layer 202. Each of the space members 203 has contact with the end surface of the electric resistance adjusting layer 202 and the conductive supporting body 201. Therefore, long-period reliability is improved, compared to the tape-based space holding member. In addition, the gap can be accurately controlled by the simultaneously process (eliminating process) of the electric resistance adjusting layer 202 and the space members (space holding members) 203.
In such a charging member 210, the space members or space holding members 203 and the electric resistance adjusting layer 202 comprise a different material, each other, in consideration of fixing toner. However,. ionic conductive agent is used as resistance adjusting agent of the electric resistance adjusting layer 202, so the water-absorbing property of the electric resistance adjusting layer 202 is increased. Therefore, the electric resistance adjusting layer 202 absorbs moisture at high temperature and high moisture, and the measurement of electric resistance adjusting layer 202 is fluctuated. The space members 203 in the charging member 210 comprise olefin series resin, so the insulation property and toner fixing resistance are improved. However, this space members (space holding members) 203 have a small amount of measurement fluctuation at high temperature and high moisture, compared to the electric resistance adjusting layer 202. Therefore, the gap formed between the charging roller and image carrier at high accuracy is fluctuated by environmental fluctuation.
In order to solve such a problem, the present inventors have proposed a conductive member 310. As shown in FIG. 11, the conductive member 310 comprises a conductive supporting body 301, an electric resistance adjusting layer 302 formed on the conductive supporting body 301 and space holding members 303 disposed in both ends of this electric resistance adjusting layer 301. The electric resistance adjusting layer 302 comprises step portions or step portions having one step or more provided in the vicinity of both ends. The step portions are disposed in both ends direction, and the step portions having one step or more are disposed in the central direction. Each of the space holding members 303 has contact with two surfaces or more comprising the step portion of the electric resistance adjusting layer 302 to be fixed thereto (reference to JP2005-019517A).
However, in the conductive member 310, if a cutting process, grinding process and the like are performed onto the surfaces of thin space holding members 303, the space holding members 303 drop out of the electric resistance layer 302 or are deformed by the stress of the cutting tool. Therefore, the gap between the conductive member and the image carrier is fluctuated.